Conventional scales normally employ a series of levers and linkages to amplify the relatively small movement of the scale platform into a large mechanical movement of a pointer. Such devices are subject to lost motion and backlash as well as friction in the bearings and therefore, are relatively inaccurate and make it difficult to obtain repeatable readings.
Electronic scales have been provided that utilize a load cell operating on the principle of a strain gauge to develop an electrical analog of the load on the platform. Load cell devices have a very high load rate. That is, the member to which the strain gauges are applied actually flexes only a very small physical distance over the load cells full capacity range. From a pure scale design criterion, it is desirable to connect such a load cell directly to the platform to minimize the interconnecting mechanism and thereby fully take advantage of the high potential accuracy of such load cells. However, when directly connected to the platform, the load cell becomes highly susceptible to overload damage. Since the platform is directly connected to the load cell, and since very little physical motion accompanies a full range of operational loads, it becomes impractical to use mechanical stops to prevent overload damage. A slight misalignment of the mechanical stops or tilting of the scale platform, for example, could cause the stops to engage the platform with a normal operational load and thereby produce false readings. Alternatively, if the mechanical stops are positioned too far away from the platform, the high load rate of the load cell may permit overload damage to the load cell prior to the stops coming into effect. Thus, many prior art electronic scales have resorted to the device of a linkage or lever arm arrangement whereby the load cell is subjected to only a fraction of the motion of the platform thereby permitting substantial physical travel of the platform prior to an overload condition and threby making it possible to use mechanical stops effectively. However, the use of lever arms or linkages makes such designs subject to many of the same deficiencies associated with prior art mechanical scale configurations.
Direct coupling of a load cell to the platform has also been hampered by the sensitivity of the load cell to the alignment of the forces transferred to it. In conventional directly coupled designs, an off-center load on the platform will produce an inaccurate reading and the load cell may be damaged by horizontal forces inadvertently applied to the platform.
Therefore, it is desirable to have a load cell scale wherein the load cell is directly coupled to the platform in such a manner as to provide overload and alignment protection. Such a design is particularly desirable wherein it incorporates a low maintenance structure, requiring a minimum of compensation, and producing repeatable readings.